1. Field of the Invention
The invention relates generally to marine propulsion devices such as outboard motors and stern drive units. More particularly, the invention relates to hydraulic trim and tilt mechanisms for vertically raising and lower the propulsion units of such marine propulsion devices. Still more particularly, the invention relates to hydraulic circuitry for such hydraulic trim and tilt mechanisms.
2. Reference to the Prior Art
In the past, because of the presence of a piston rod within a hydraulic cylinder, a given amount of piston movement resulted in a greater inflow or outflow at the non-piston rod end of the cylinder as compared to the inflow or outflow at the piston rod end of the cylinder.
In prior hydraulic circuits, the pump operated to supply hydraulic oil under pressure to the rod end of the cylinder during down or in-trimming. However, some of the hydraulic oil discharged from the pump was returned through a pressure relief valve to the sump because, for a given amount of piston movement, the flow from the non-piston rod end of the trim cylinder was greater than the flow to the piston rod end of the trim cylinder. Thus, not all of the oil under pressure discharged from the pump was used for in-trimming with the result that a longer time duration occurred during in-trimming than would have resulted if all of the hydraulic fluid leaving the pump was supplied to the rod end of the cylinder. Such slowness in action enhanced the possibility of the propulsion unit driving down and hitting or striking the piston rod because of the slowness of the in-trimming operation.
Attention is directed to the following U.S. Pat Nos:
______________________________________ 4,064,824 Hall, et al. December 27, 1977 4,096,820 C. B. Hall June 27, 1978 4,687,449 W. G. Fenrich August 18, 1987 4,781,632 Uchida, et al. November 1, 1988 ______________________________________